1. Field of the Invention
The present invention relates to a dye transfer type thermal printing sheet from which a dye is transferred onto a color developing layer of an image-receiving sheet to form an image and which is for multiple use where the same part of the printing sheet is used repeatedly.
2. Description of the Related Art
Dye transfer type thermal printing which uses dyes having high sublimation properties is a kind of full-color recording system which enables printing with concentration gradation at each recording dot. Since the printing sheet is expensive, many attempts on multiple use of the printing sheet have been reported in, for example, (1) "Partially Reusable Printing Characteristics of Dye Transfer Type Thermal Printing Sheets" (Papers for the 2nd Nonimpact Printing Technology Symposium (1985), pages 101-104); (2) "Study on Sublimation Type Film for Multiple Recording" (Preprints for 1986 Annual Meeting of Image Electronics Society); (3) Japanese Patent Kokai Publication No. 27291/1988; and (4) "Multi-Usable Dye Transfer Sheets" (Preprints for the 61st Study and Discussion Meeting of the Society of Electrophotography, pages 266-269).
The multiple recording modes are classified into two one of which is a simple repeating mode in which the same part of the printing sheet is used N times and the other of which is an n-times relative speed mode in which a supply speed of the printing sheet is adjusted to 1/n time of that of the image-receiving sheet so that n-times multiple printing is performed. The above four conventional arts (1) through (4) relate to the multiple recording by the relative speed mode. Since a fresh part of the printing sheet is always supplied in the relative speed mode, the substantial number of repeat is larger in the relative speed mode than in the simple repeating mode.
The relative speed mode requires some measure to keep lubricity between the printing sheet and the image-receiving sheet. The conventional arts (1) and (3) used spherical spacer particles or solid lubricants such as molybdenum disulfide to keep the lubricity between the printing sheet and the image-receiving sheet. In the above conventional art (2), the recording by the relative speed mode is achieved by closely contacting the printing sheet and the image-receiving sheet. However, the report (2) is silent on a measure for keeping the lubricity. In the report (4), decrease of a dye concentration in a dye layer surface is prevented by controlling easiness of diffusion of the dye in the dye layer or the color developing layer of the image-receiving sheet or by forming a concentration distribution in a direction of thickness of the dye layer. Thereby, the quality of the multiple printing is improved. In addition, a lubricant is added to the dye layer and the color developing layer of the image-receiving sheet.
To realize the full-color image printing having the same quality as a general printing (one-time printing), it is necessary to achieve the same saturated print density (about 1.5 to 1.8) as that in the general printing and small variation of the printing concentration against the same recording energy during multiple printing so as to avoid the influence of print hysteresis.
In the conventional art (1), once a sufficient amount of the dye for multiple printing is used, the printing characteristics are satisfied. However, since a space is kept between the printing sheet and the image-receiving sheet to give lubricity for relative speed traveling and to determine a printing rate by the sublimation step, the dye should be one having a high sublimation property. Although, in the conventional art (2), a weather durabili a low sublimation property can be used because of contact diffusion printing, the print density against the same recording energy greatly decreases as the number of prints increases even if the sufficient amount of the dye for multiple printing is supplied. As the result, the saturated print density achieved by the multiple printing does not reach a practical level. In the conventional art (3), as in the conventional art (1), the print density decreases in comparison to the system having no spacer. When the particle size of the spacer is small, decrease of the recording concentration caused by increase of the relative speed ratio cannot be ignored.
Contrary to the above, the conventional art (4) uses a dye transfer type printing sheet comprising a base sheet and a dye layer containing a dye in such concentration distribution that a weight concentration on the layer surface side is lower than that on the base sheet side, whereby it is possible to use the same part of the printing sheet many times in the contact diffusion printing. However, when a layer containing the dye in a lower concentration and an oil-soluble resin is coated in the form of a solution in an organic solvent on an already coated layer containing the dye in a high concentration, the latter is dissolved so that it is difficult to keep the low dye concentration on the surface side. Therefore, the expected high multiple printing performance is not necessarily achieved. Since no spherical spacer is used, the printing sheet tends to weld or stick easily to the image-receiving sheet, and it is difficult to perform the relative speed mode printing. To perform the relative speed printing, a lubricant such as a fatty acid derivative having a comparatively low molecular weight or a wax and silicone oil which is in the liquid state at room temperature is added. However, the lubricant induces recrystallization of the dye on the dye layer surface. Therefore, the printing sheet has poor storage stability, or the lubricant is transferred to the surface of the image-receiving sheet so that the weather durability of the printed image is deteriorated.